The present invention relates to an ink ejection printing apparatus for an ink jet printer. Such a printer comprises an ink ejection nozzle in which is provided an ultrasonic vibrator. Application of ejection or drive pulses to the vibrator causes an ink jet ejected from the nozzle to be atomized into drops or droplets. The ink drops are electrically charged by an electrode. A deflection voltage is applied to deflection electrodes which deflect the charged droplets onto paper for printing. Where it is desired not to print a dot, no charging voltage is applied and the ink droplets are caught by a gutter. A prior art example of such an ink ejection printing apparatus is disclosed in IBM Technical Disclosure Bulletin Vol. 16, No. 12, May 1974, Japanese patent publication No. 47-43450 and Japanese patent application disclosure No. 50-46450.
One problem in a system of the present type is to synchronize application of the charging pulses applied to the charging electrode with the position of the ink drops. The charge will be optimum only if the charging pulses are applied to the charging electrode at the time the ink drops are adjacent to the electrode. Synchronism can be achieved by providing a sensing electrode downstream of the charging electrode for sensing the amount of charge on the ink drops and varying the phase between ink ejection pulses and charging pulses until a desired charge value is achieved. This is known as a phase sweep operation and is disclosed in Japanese patent publication No. 47-43450 and Japanese patent application disclosure No. 50-60131.
Another problem is in adjusting the amount of deflection of the ink jet to an optimum value. If the deflection is too great or too small, the printed image will be distorted, particularly enlarged or reduced in relation to the main scan feed pitch. This can, in extreme cases, produce an unintelligible image. The problem is compounded by the fact that the deflection is a function of a number of variables, including the charge on the ink drops, the mass of the ink drops, the deflection voltage, the spacing between the deflection electrodes and the ejection velocity of the drops. Mere adjustment of the ink drop charge using the phase sweep operation cannot result in a predetermined amount of deflection since the deflection also depends on the other variables.
Another problem involves the printing density, or the darkness of the printed characters or pattern. If the mass of the ink drops in too high, the printing density will be excessive and vice-versa. The printing density varies in accordance with the output pressure of an ink ejection pump and the temperature of the ink.
As the ink temperature increases, the mass of the ink drops decreases and the ejection velocity increases. As the pump pressure increases, the ejection velocity increases. Therefore, there is a correlation between the ink ejection velocity and the printing density. An increase in ink temperature makes the ink thin and decreases the printing density and an increase in pump pressure increases the amount of ink per unit area and increases the printing density. However, the printing density will have a desired optimum value at a corresponding value of ink ejection velocity.